Nonlinear Filter and Transmission Circuit

1. A nonlinear filter for inserting, based on an I signal (an in-phase signal) and a Q signal (a quadrature-phase signal) inputted into the nonlinear filter, a pulse into the I signal and the Q signal, and outputting resultant I and Q signals, the nonlinear filter comprising: a determination unit that determines, based on the inputted I and Q signals, whether or not to insert a pulse; a rotation detector that detects, based on the inputted I and Q signals, a rotation direction of variations of the I and Q signals on an IQ plane with respect to the origin on the IQ plane, wherein the rotation direction is represented based on whether the variations of the I and Q signals are in an anticlockwise direction or in a clockwise direction with respect to the origin on the IQ plane; a pulse generator that generates, when the determination unit determines to insert a pulse, a pulse of which at least one of the direction and the magnitude is determined in accordance with at least the detected rotation direction; and an adder that inserts the pulse into the inputted I and Q signals and outputs resultant I and Q signals.

2. The nonlinear filter according to claim 1 , wherein the determination unit compares magnitudes of the I and Q signals with a predetermined threshold value, and determines based on a result of the comparison whether or not to insert the pulse.

3. The nonlinear filter according to claim 1 , wherein the adder is configured to have multiple delay devices and multiple adders therein that are alternately connected, and the determination unit determines whether or not to continue pulse insertion, based on a signal outputted from at least one of the multiple adders.

4. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, and the symbol closest to the origin is present within a circle that centers around the origin and that has a first radius, the determination unit determines to: insert, when the detected rotation direction is a first direction, a pulse having a first vector whose starting point is an intersection point of a first straight line passing through the first and second symbols and a second straight line orthogonal to the first straight line and passing through the origin, and whose end point is one of intersection points of the second straight line and the circle having the first radius, which one of the intersection points is closer to the starting point than the other intersection point; and insert, when the detected rotation direction is a second direction, a pulse having a second vector whose starting point is the intersection point of the first and second straight lines and whose end point is one of intersection points of the second straight line and a circle that centers around the origin and that has a second radius different from the first radius, which one of the intersection points is closer to the starting point than the other intersection point.

5. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, the determination unit determines to: insert, when the symbol closest to the origin is present within a circle that centers around the origin and that has a first radius and the detected rotation direction is a first direction, a pulse having a first vector whose starting point is an intersection point of a first straight line passing through the first and second symbols and a second straight line orthogonal to the first straight line and passing through the origin, and whose end point is one of intersection points of the second straight line and the circle having the first radius, which one of the intersection points is closer to the starting point than the other intersection point; and insert, when the symbol closest to the origin is present within a circle that centers around the origin and that has a second radius different from the first radius and the detected rotation direction is a second direction, a pulse having a second vector whose starting point is the intersection point of the first and second straight lines and whose end point is one of intersection points of the second straight line and the circle having the second radius, which one of the intersection points is closer to the starting point than the other intersection point.

6. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, and the symbol closest to the origin is present within a circle that centers around the origin and that has a first radius, the determination unit determines to: insert, when the detected rotation direction is a first direction, a pulse having a first vector whose starting point is an intersection point of a first straight line passing through the first and second symbols and a second straight line orthogonal to the first straight line and passing through the origin, and whose end point is one of intersection points of the second straight line and the circle having the first radius, which one of the intersection points is closer to the starting point than the other intersection point; and insert, when the detected rotation direction is a second direction, a pulse having a second vector whose starting point is the intersection point of the first and second straight lines and whose end point is one of the intersection points of the second straight line and the circle having the first radius, which one of the intersection points is more distant from the starting point than the other intersection point.

7. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, and the symbol closest to the origin is present within a circle that centers around the origin and that has a first radius, the determination unit determines to: insert, when the detected rotation direction is a first direction, a pulse having a first vector whose starting point is an intersection point of a first straight line passing through the first and second symbols and a second straight line orthogonal to the first straight line and passing through the origin, and whose end point is one of intersection points of the second straight line and the circle having the first radius, which one of the intersection points is closer to the starting point than the other intersection point; and insert, when the detected rotation direction is a second direction, a pulse having a second vector whose starting point is the intersection point of the first and second straight lines and whose end point is one of intersection points of the second straight line and a circle that centers around the origin and that has a second radius different from the first radius, which one of the intersection points is more distant from the starting point than the other intersection point.

8. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, and a value obtained by dividing a distance between the first and second symbols by a distance, from the origin, of the symbol closest to the origin is greater than a predetermined threshold value, the determination unit determines to: insert, when the detected rotation direction is a first direction, a pulse having a first vector whose magnitude is a result of subtracting the threshold value from the value obtained by the dividing and then multiplying a predetermined first constant by a value resulting from the subtracting, and whose direction is one of directions orthogonal to a straight line passing through the first and second symbols, which one of the directions is a direction from the origin toward the straight line; and insert, when the detected rotation direction is a second direction, a pulse having a second vector whose magnitude is a result of subtracting the threshold value from the value obtained by the dividing and then multiplying a predetermined second constant different from the first constant by the value resulting from the subtracting, and whose direction is one of the directions orthogonal to the straight line passing through the first and second symbols, which one of the directions is the direction from the origin toward the straight line.

9. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, the determination unit determines to: insert, when a value obtained by dividing a distance between the first and second symbols by a distance, from the origin, of the symbol closest to the origin is greater than a predetermined first threshold value and the detected rotation direction is a first direction, a pulse having a first vector whose magnitude is a result of subtracting the first threshold value from the value obtained by the dividing and then multiplying a predetermined first constant by a value resulting from the subtracting, and whose direction is one of directions orthogonal to a straight line passing through the first and second symbols, which one of the directions is a direction from the origin toward the straight line; and insert, when the value obtained by dividing the distance between the first and second symbols by the distance, from the origin, of the symbol closest to the origin is greater than a predetermined second threshold value different from the first threshold value and the detected rotation direction is a second direction, a pulse having a second vector whose magnitude is a result of subtracting the second threshold value from the value obtained by the dividing and then multiplying a predetermined second constant different from the first constant by a value resulting from the subtracting, and whose direction is one of the directions orthogonal to the straight line passing through the first and second symbols, which one of the directions is the direction from the origin toward the straight line.

10. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, and a value obtained by dividing a distance between the first and second symbols by a distance, from the origin, of the symbol closest to the origin is greater than a predetermined threshold value, the determination unit determines to: insert, when the detected rotation direction is a first direction, a pulse having a first vector whose magnitude is a result of subtracting the threshold value from the value obtained by the dividing and then multiplying a predetermined first constant by a value resulting from the subtracting, and whose direction is one of directions orthogonal to a straight line passing through the first and second symbols, which one of the directions is a direction from the origin toward the straight line; and insert, when the detected rotation direction is a second direction, a pulse having a second vector whose magnitude is a result of subtracting the threshold value from the value obtained by the dividing and then multiplying a predetermined second constant different from the first constant by the value resulting from the subtracting, and whose direction is one of the directions orthogonal to the straight line passing through the first and second symbols, which one of the directions is a direction from the straight line and away from the origin.

11. The nonlinear filter according to claim 1 , wherein in a case where symbols that are obtained by oversampling the I and Q signals indicate a symbol transition from a first symbol to a second symbol, and one of the first and second symbols is, during a predetermined time period, a symbol closest to the origin on the IQ plane and the other one of the first and second symbols is, during the predetermined time period, a symbol second closest to the origin on the IQ plane, the determination unit determines to: insert, when a value obtained by dividing a distance between the first and second symbols by a distance, from the origin, of the symbol closest to the origin is greater than a predetermined first threshold value and the detected rotation direction is a first direction, a pulse having a first vector whose magnitude is a result of subtracting the first threshold value from the value obtained by the dividing and then multiplying a predetermined first constant by a value resulting from the subtracting, and whose direction is one of directions orthogonal to a straight line passing through the first and second symbols, which one of the directions is a direction from the origin toward the straight line; and insert, when a value obtained by dividing the distance between the first and second symbols by the distance, from the origin, of the symbol closest to the origin is greater than a predetermined second threshold value different from the first threshold value and the detected rotation direction is a second direction, a pulse having a second vector whose magnitude is a result of subtracting the second threshold value from the value obtained by the dividing and then multiplying a predetermined second constant different from the first constant by a value resulting from the subtracting, and whose direction is one of the directions orthogonal to the straight line passing through the first and second symbols, which one of the directions is a direction from the straight line and away from the origin.

12. A transmission circuit for modulating an I signal and a Q signal inputted into the transmission circuit to generate a transmission signal, and outputting the transmission signal, the transmission circuit comprising: the nonlinear filter according to claim 1 , into which the I and Q signals are inputted; a coordinate system converter that converts output signals from the nonlinear filter into a phase signal and an amplitude signal; an angle modulator that performs angle modulation based on the phase signal and thereby generates an angle-modulated signal; and an amplitude modulator that performs, based on an amplitude represented by the amplitude signal, amplitude modulation on the angle-modulated signal and thereby generates the transmission signal.

13. A radio communication device comprising: a transmitter that includes the transmission circuit according to claim 12 ; a receiver; an antenna; and one of a duplexer and an antenna switch, wherein the transmitter and the receiver are connected to the antenna via the one of the duplexer and the antenna switch.

14. An integrated circuit that integrates, at least partly, functions of the radio communication device according to claim 13 , the integrated circuit integrating at least one of: a function of the determination unit that determines based on the inputted I and Q signals whether or not to insert the pulse; a function of the rotation detector that detects, based on the inputted I and Q signals, a rotation direction of variations of the I and Q signals on the IQ plane with respect to the origin of the IQ plane; a function of the pulse generator that generates a pulse in accordance with the detected rotation direction; and a function of the adder that inserts the pulse into the inputted I and Q signals and outputs resultant I and Q signals.

15. A method for a processor to perform a nonlinear filtering process on an I signal (an in-phase signal) and a Q signal (a quadrature-phase signal) inputted into the processor, the method comprising: a determination step at which the processor determines based on the inputted I and Q signals whether or not to perform pulse insertion; a rotation direction detection step at which the processor detects, based on the inputted I and Q signals, a rotation direction of variations of the I and Q signals on an IQ plane with respect to the origin of the IQ plane, wherein the rotation direction is represented based on whether the variations of the I and Q signals are in an anticlockwise direction or in a clockwise direction with respect to the origin on the IQ plane; a pulse generation step at which the processor generates a pulse in accordance with the detected rotation direction; and an addition step at which the processor inserts the pulse into the inputted I and Q signals, and outputting resultant I and Q signals.